1. Field of the Invention
The present invention relates generally to an acoustic combination responsive to an acoustic wave, and more particularly, to an acoustic combination that can be utilized as a dynamic pressure sensor and a microelectromechanical systems (MEMS)-based acoustic array that utilizes the acoustic combination.
2. Background
As aircraft noise regulations become more stringent, the need for modeling and measuring aircraft noise phenomena becomes more important. In order to intelligently design quieter aircraft, the physical mechanisms of noise generation should be understood and any theoretical or computational noise model should be experimentally validated. One validation method is the comparison of the theoretical and measured acoustic far-field pressures. However, single microphone measurements of aeroacoustic sources in wind tunnels are hampered by poor signal to noise ratios that arise from microphone wind self-noise, tunnel system drive noise, reverberation, and electromagnetic interference. In addition, a single microphone cannot distinguish pressure contributions from different source locations. The need for more precise noise source characterization and localization has driven the development of advanced sound field measurement techniques. In particular, the development and application of directional (phased) microphone arrays have been documented as a means to localize and characterize aeroacoustic sources in the presence of high background noise.
Although knowledge of the acoustic field does not uniquely define the source, localization of a source and analyses of the spatial and temporal characteristics of its far-field radiation can provide insight into noise generation mechanisms. Modern acoustic arrays used in wind tunnel studies of airframe noise are typically constructed of moderate numbers (≦100) of instrumentation grade condenser microphones, and range in aperture size from several inches to several feet. Data collection, followed by extensive post-processing has been used to implement various beamforming processes, including conventional beamforming, array shading, shear-layer corrections, adaptive methods, etc. The resulting data files can be over 500 GB in size and require up to an hour of post-processing per data set.
Greater numbers of microphones in an array can improve the ability to characterize a sound field. A greater number of microphones enhances the signal to noise ratio of an array, defined as the array gain, given (in dB) by 10*log(M), where M is the number of microphones. In addition, a large number of microphones may be used to extend the frequency range of an array. The spatial resolution of an array is related to the product kD, where k=ω/c is the acoustic wavenumber, ω is the radian frequency, c is the speed of sound, and D is the aperture size. Thus, a larger aperture is needed to improve the spatial resolution of an array, of most concern at low frequencies. In contrast, the intersensor spacing must be kept less than one-half of the smallest wavelength of interest (highest frequency) to avoid spatial aliasing. The feasibility of scaling the current technology to multiple arrays with large numbers (hundreds or thousands) of microphones is limited by the cost per channel (microphone, amplifier, data acquisition), data handling efficiency (acquisition capabilities, signal processing complexity, storage requirements), and array mobility (size, weight, cabling). In addition, experiments performed in large wind tunnels are costly and require extensive setup. Thus, an array system that provides near real-time output would be advantageous.
Thus, there is a need to develop a new acoustic array system that, among other applications, can be utilized for aeroacoustic measurement.